The present invention relates to virtual terrain architectures for three-dimensional visualization.
Virtual terrain databases for three-dimensional visualization are typically based on some type of map projection. Some common map projections used are Universal Transverse Mercator (UTM), Geodetic, Lambert Conic, and Flat Earth among several others. A database projection warps the curved surface of the earth onto a flat X-Y plane to create a local Cartesian coordinate system that simplifies the mathematics of a simulation.
Projection-based database architectures typically subdivide the X-Y extents of a desired geographic region into small, uniformly-sized, rectangular areas commonly referred to as tiles or patches. The geographic coverage of the entire database is then a mosaic of these tiles. Database tiles typically represent a unit of database compiler output, a load module or unit of paging by the runtime system, and/or a unit of culling by the runtime system.
One problem with such projection-based systems can be seen by looking at a world map. On a world map, areas that are near the north or south poles are distorted when projected into this two dimensional flat space as opposed to a three dimensional representation.
A more ideal and accurate representation of the earth's surface is an earth-centered, earth-fixed coordinate system that accurately represents curvature of the earth and deals with ordinate axis convergence at the poles. Such a coordinate system is often referred to as geocentric. Projection-based database generating systems do not create geocentric data, since these databases store data in two-dimensional space. Similarly, the projection-based image rendering systems can not render data in a geocentric coordinate system due to the incompatibility of data types. Furthermore, since there is no X-Y plane onto which the database has been projected in a geocentric coordinate system, classic tiling schemes used for database paging do not map well into the geocentric environment.